The use of power electronics to supply induction motors
is widely spreading out due to significant advantages in terms of speed
and torque control of electrical machines. The most common are
ASD (Adjustable Speed Drives) systems,
lead by PWM (Pulse Width Modulation)
techniques.

Unfortunately, it has been proved that this kind of
power supply can cause overstress on electrical insulation in comparison
to the sinusoidal 50/60 Hz voltage supply [1-13], thus decreasing
drastically life and reliability of controlled motors.

The reliability of winding insulation, as it occurs for
other insulation systems, can be investigated by Partial Discharge (PD)
analysis.

Actually, this research is focused on:

on-line and off-line investigation of voltage distribution along
windings;

Off-line testing

Figure 1 shows the off-line testing apparatus, TechSquare,
developed by
TechImp. It is composed of a HV squarewave generator controlled in
amplitude, frequency, rise time and duty cycle. The voltage waveforms are
acquired by the PDSolver system developed by
TechImp. The motor
under testing is usually supplied with open star, one phase connected to
the inverter and the other two grounded. Increasing the supply voltage, it is
possible to reach the partial discharge inception voltage (PDIV) of the
insulation system.

On-line
testing

On-line measurements on motors supplied by electronic
power converters is another aspect of the research. An inverter bench is
used for motor supply with the purpose of understanding the differences
between on-line and off-line configurations, e.g. EM disturbs (sparks,
mobile phones, etc.) and voltage distribution along motor winding.